Metasurfaces, which are the 2D version of metamaterials, have revolutionised compact optics. Using subwavelength periodic nanostructured dielectrics, the refractive index and absorption properties of metasurfaces can manipulate light to a degree surpassing conventional bulk materials. Using metasurfaces, the phase, polarization, spin (for circularly polarised light), amplitude and wavelength of light can all be arbitrarily tailored to imitate a lens, which we refer to as a metalens (ML). MLs allow a larger choice of materials for optical components and have five major advantages over traditional refractive lenses – superior resolution, miniaturisation, lighter weight, multifunctionality and cost. In recent years, numerous metasurfaces with useful functionalities have been proposed, and although novel in their approach they still have very few real-world applications. One such application is within infrared laser systems, which have real-world use such as laser designators.
In this work, we demonstrate polarisation-insensitive metalenses working at λ = 1064 nm, with a d = 1 mm aperture size and four different F-numbers (f# = 0.5 - 5). The lenses are made using amorphous silicon (a-Si) pillars on top of a fused silica substrate, in order to function with a high efficiency (>60%) and little loss – where previous metal-based (plasmonic) metalens devices suffered from low efficiency (<10%) and high loss. The a-Si pillars range from 70-360 nm diameter atop a fused silica substrate, which are fabricated using electron beam lithography (EBL) and reactive ion etching processes.
The characterised lenses are shown to have almost diffraction-limited focal spot sizes, agreeing with the theoretical values of λ.f/d, and focussing efficiencies of 60%. Furthermore, we have designed large area lenses with aperture d = 10 mm, where the number of pillars per lens exceeds 550 million. By using an efficient Python script, we are able to make these 100 mm2 samples with just 14 hours of EBL writing time. The 10 mm lenses have focal lengths of 5 mm and 20 mm (F-numbers of 0.5 and 2 respectively). Such large area lenses are of considerable interest to many commercial applications where superior resolution and a light weight are beneficial, including laser designators/targeting, airborne and aerospace applications, as well as handheld devices.